Some observations on the design and construction of wet ......mechanical soil mixing and jet grouting technologies – its primary advantages being the facility to work effectively

Proceedings of the XVII ECSMGE-2019 Geotechnical Engineering foundation of the future

ISBN 978-9935-9436-1-3 © The authors and IGS: All rights reserved, 2019 doi: 10.32075/17ECSMGE-2019-0233

IGS 1 ECSMGE-2019 - Proceedings

Some observations on the design and construction of

wet soil mixing in the UK Quelques observations sur la conception et la construction de

mélanges de sol humide au Royaume-Uni A. O’Brien

GE Solutions Consulting Ltd, Whitburn, Scotland

ABSTRACT: Mass soil mixing and deep soil mix columns are a versatile ground improvement technology for

marginal and brownfield sites. Dry soil mixing is relatively common in the UK for improvement of ground with

very wet and/or organic material. Wet mixing is less commonplace and involves introduction of a fluid grout

with simultaneous disaggregation of the soil with a rotating mixing tool. This paper presents the results of strength

verification testing carried out across multiple projects in the UK covering differing soil types with varying

project specification criteria. Some conclusions are drawn with regards to the factors affecting strength

progression and in understanding the mechanics of the mixing process. Mixing time per unit volume of mixed

material is identified as an important parameter for mass mixing. In addition, discrete element modelling has

shown promise in understanding the mechanics of deep column mixing.

RÉSUMÉ: Les colonnes de mélange de sol en masse et de mélange de sol en profondeur constituent une

technologie polyvalente d'amélioration du sol pour les terrains marginaux et les sites contaminés. Au Royaume-

Uni, le mélange de sol sec est relativement courant pour améliorer le sol avec des matières très humides et / ou

organiques. Le mélange humide est moins courant et implique l'introduction d'un coulis fluide avec une

désagrégation simultanée du sol avec un outil de mélange rotatif. Ce document présente les résultats d’essais de

vérification de la résistance menés au Royaume-Uni dans plusieurs projets couvrant différents types de sol et

différents critères de spécification de projet. Certaines conclusions sont tirées en ce qui concerne les facteurs

influant sur la progression de la résistance et sur la compréhension des mécanismes du processus de mélange. Le

temps de mélange par unité de volume de matériau mélangé est identifié comme un paramètre important pour le

mélange en masse. En outre, la modélisation discrète éléments s’est révélée prometteuse pour comprendre les

mécanismes du mélange en colonne profonde.

Keywords: deep soil mixing; mass mixing; soft ground; ground improvement; discrete element modelling

1 INTRODUCTION

Ground improvement technologies are used

extensively in the civil engineering and building

industries to engender higher strength, lower

compressibility or improvement of other

engineering properties into native soils for the

purposes of accommodating greater load or

achieving a greater level of serviceability for a

structure than would have otherwise been

possible. Success of these methods are prevalent

in Japan, the United States of America,

Scandinavia, Great Britain and Ireland

(Munfakh, 1997; Terashi & Tanaka, 1981; Hebib

& Farrell, 2004) having been pioneered initially

and independently in Japan and Scandinavia.

B.3 - Ground reinforcement and ground improvement

ECSMGE-2019 – Proceedings 2 IGS

In extensive and deep deposits of soft ground,

deep soil mixing, traditionally encompassing the

mechanical agitation of ground with the addition

of a cementitious or lime binder, is commonplace

as an improvement method. The chemical

processes of binder introduction, (i.e. hydration

& subsequent production of primary & secondary

cementitious by-products, ion exchange &

flocculation, pozzolanic reaction and

carbonation), are well understood with well-

defined relationships between the volume of

binder introduced and the strength and/or

stiffness increase (for a given binder type or

blend). However, the mechanics of the mixing

processes are not well understood.

1.1 Soil mixed columns

Soil mixed columns find particular application in

the treatment of deep deposits of poor materials.

Typically the columns are combined with a soil

mixed load transfer platform to provide a

working formation of high bearing capacity.

Other applications include settlement reducing

techniques and cut-off walls.

Figure 1 shows a proprietary rig-mounted wet

soil mixing system known colloqially as Turbojet

(developed by Trevi Soilmec) which will be a

focus of this paper. The system involves

penetration of a mixing tool at high revolutions

per minute in conjunction with the introduction

of liquid grout under high pressure (typically in

excess of 250 bar). A typical mixing tool is

shown in Figure 2. The combination of the high

number of blade rotations per unit depth and the

disaggregation engendered by the grout under

high pressure results in complete destructuring of

the native soil and, thus, high quality mixed soil.

The system is considered to be a hydrid of

mechanical soil mixing and jet grouting

technologies – its primary advantages being the

facility to work effectively in a broader range of

soil parameters, both granular & cohesive,

including high plasticity clays and high

production rates. Wet soil mixing techniques are

sub-optimal for soils of natural moisture content

in excess of 100% where the native water content

diminishes the effect of grout addition. The

governing construction parameter is the blade

rotation number (BRN), defined in EN

14679:2005.

Figure 1. Turbojet system for deep soil mixing (cour-

tesy of Ground Developments Ltd)

Figure 2. Turbojet mixing tool (courtesy of Ground

Developments Ltd)

Some observations on the design and construction of wet soil mixing in the UK


1.2 Mass mixing

Mass mixing (see Figure 3) involves

disaggregation of the soil using an excavator-

mounted rotavating tool (see Figure 4). In

general, the principle is the same as deep columns

however mixing usually takes place within

discrete “cells” and depth is limited up to 5-7m

depending on the application and native soil.

The rotavating tool spins at high revolutions

per minute (in the order of 80-90rpm) and grout

is injected under medium pressure. The result is

a completely fluidised cell and mixing time per

cell is conjectured to be critical to homogenising

the material and engendering the required

strength.

Figure 3. Mass mixing system (courtesy of Ground

Developments Ltd)

Figure 4. Mass mixing tool (courtesy of Ground De-

velopments Ltd)

2 BACKGROUND TO PROJECTS

The projects which are the subject of this paper

cover northern England and Scotland as shown in

Figure 5. Table 1 compares and contrasts the

projects in terms of ground conditions and soil

mixing types. In all cases, groundwater was near

(within 1m) commencement level.

Figure 5. UK project locations

Project

location

Indicative soil type Mixing type /

depth

Walney Tidal flats, very soft,

sensitive CLAY

Deep columns

up to 25m deep

Dundee Infilled quarry,

uncontrolled granular

fill & waste (ash)

Deep columns

up to 17m deep

Airdrie Granular Made

Ground with PEAT

lenses up to 1.5m

thick

Mass mixing

up to 4m deep

Stonehouse Very soft, sensitive

CLAY

Mass mixing

up to 5m deep

Aberdeen Very soft SILT with

marine influence

Mass mixing

up to 6m deep

Table 1. Summary of project conditions



3 OBSERVATIONS

3.1 Deep soil mixing

While the focus of this paper is on the laboratory

strength testing for routine quality control of soil

mixing, it is important to note that laboratory tests

should be augmented by visual-manual

inspection where appropriate. This can be

undertaken by exposing mixed material (see

Figure 6 which shows the head of a soil mixed

column exposed) and through extraction of rotary

cores post-construction (see Figure 7).

Figure 6. Exposed soil mixed column (courtesy of

Ground Developments Ltd)

Figure 7. Partial rotary core through soil mix col-

umn (courtesy of Ground Developments Ltd)

3.1.1 Density

In routine strength testing, undertaken as

unconfined compressive strength testing of

manufactured cubes of samples extracted from

site, density of the sample is normally measured.

Observations have not demonstrated any reliable

correlation of strength and sample density. In

Figures 6 & 7 below, strength and density are

compared for 14-day and 28-day cured samples

from the Walney and Dundee sites.

The Dundee data has inherently lower density

owing to the nature of the native mixed soil,

being of granular, ash & waste composition and

this is reflected in the mixed material. However,

this has not affected the overall strength gain.

Both sets of results express similar strengths

(both sites had equal target design strengths).

Figure 8. 14-day strength-density plots

Figure 9. 28-day strength-density plots



3.1.2 Laboratory strength

Histograms are presented in Figures 8 & 9 for the

strength measurement from the Dundee &

Walney sites for 14-day and 28-day strengths

respectively. The results indicate a normal

distribution but with a hint of right skew on the

7-day results. Standard deviations for the 14-day

& 28-day results were 0.34MN/m2 & 0.25MN/m2

respectively – the target 28-day strength was

1.0MN/m2.

The Dundee (predominantly granular) data

cumulatively expressed more rapid strength gain.

However, at 28 days the Walney (predominantly

cohesive) data had exceeded the Dundee strength

cumulatively.

Figure 10. Histogram of 14-day strength test results

Figure 11. Histogram of 28-day strength test results

The mean 14-, 28- & 48-day strengths are

presented in Figure 12 below. The data was found

to fit well with the relationship of Åhnberg

(2006) as follows:

𝑞𝑡𝑞28

≈ 0.3 ∙ ln 𝑡

Where: qt is the strength at time t

q28 is the strength at 28-days

However it should be noted that the sample size

for 48-days is comparably small. For statistical

completeness, the associated box plots for the

Dundee & Walney strength data are presented in

Figures 13 & 14 below respectively.

Figure 12. Strength gain of mixed material from col-

umn samples

Figure 13. Box plot of strength results for Dundee



Figure 14. Box plot of strength for Walney

3.2 Mass mixing

The mass mixing system is a versatile tool with

applications in both permanent & temporary

works. The range of projects considered here

(Airdrie, Stonehouse & Aberdeen) included

bearing capacity improvement for a retaining

wall, car-park and large silo tanks. Other

applications include stabilisation of deep

extremely soft soil for workability, cut-off walls

for groundwater & ground gas and improvement

of passive soils in cofferdams for temporary

excavation support.

The design of the rotavating head itself is

specific to soil type. Figure 15 below shows a

head more suited to cohesive soils where as the

head shown in Figure 4 is more suited to granular

materials.

Figure 15. Alternative mixing head (courtesy of

Ground Developments Ltd)

3.2.1 Mixing time

Experience has shown that mixing time is critical

to the success of mass mixing procedures.

Specifically, mixing time per unit volume of

material in each cell is conjectured to be a critical

measure to ensure adequate strength gain and

homogenity of the mixed material.

In the mass mixing projects discussed here, the

veracity of this perception is examined from full-

scale project data. Mixing time is plotted against

strength gain in Figures 16 & 17 below for 7/14-

day and 28-day strength measurements.

However, given that the mix design is markedly

different for the Aberdeen project, these results

have been normalised by q* (considered as the

mean result plus one-half of a standard deviation

from the mean as an upper bound of the results)

and presented in Figure 18.

Figure 16. 7-day to 14-day strength & mixing time

results for mass mixed sites

Figure 17. 28-day strength & mixing time results for

mass mixed sites



The normalised plot shows that mixing time

potentially has an optimal minimum of between

1.5 and 1.75 minutes. In statistical terms, the

sample range is too narrow to propose a

reasonable model but there is evidence of a peak

(and thus optimal) mixing time where strength

gain is maximised.

The data strongly indicates that mixing time

less than 1.0 minutes has a detrimental effect on

strength gain. Equally, there is evidence of a

tapering off of strength gain with mixing time in

excess of 1.5 to 1.75 minutes.

These observations are very important in terms

of developing project specifications where wet

mass mixing is proposed. Mixing time per unit

volume should be considered as a primary control

parameter. Soil mixing does not currently have a

standardised specification in the UK. Typically,

projects where soil mixing is proposed will have

a bespoke specification identifying geotechnical

properties for the mixed material. EN

14679:2005 is normally proposed as a guiding

code of practice. While the BRN is a useful

control parameter for deep columns, there is no

equivalent control parameter for mass mixing

appartus. Mixing time per unit volume appears to

be a purposeful measure to this end.

Further work and data collection over a wider

range of mixing times is needed to form a firmer

view on the precise relationship between mixing

time and strength gain.

Figure 18. Relationship of mixing time and strength

development

4 FUTURE WORK & RESEARCH

4.1 Data development

The observations presented here are based on a

limited project set, though reflect a broad range

of soil types and project applications. It is hoped

that the data will be continuously augmented in

order to develop understanding of the

mechanisms of strength development in mixed

material.

The mechanisms controlling strength gain in

deep columns are better understood (e.g. BRE,

2002) however, the mechanisms controlling the

success of mass mixing applications are less well

understood. A broader database of soil types,

project applications and field & laboratory

measurements is needed to develop firm theories

and mathematical models of the governing

mechanics.

4.2 DEM modelling

The excellent work of Larsson (2003) outlines the

state-of-the-art in the understanding of mixing

processes for soil mixing applications.

Some recent work by O’Brien (2018) studying

the mixing mechanisms using discrete element

modelling (DEM) has provided some useful

qualitative insight into the mechanisms

governing the BRN. The DEM model (see Figure

19) uses particle trace to track individual

elements during the mixing process and chart the

particle behaviour (see Figure 20). The

qualitative conclusions of the study outline how

higher BRNs cause cyclical migration of the

particles in the plane of rotation about the tool but

also laterally paerpendicular to the tool, thus

inducing a better quality of mix.

Expansion of such studies is required to

develop a numerical framework for both the

qualitative & quantitative understanding of

mixing mechanisms and to supplement the

empirical observations derived from field and

laboratory data as highlighted in this paper.



Figure 19. Particle trace during soil mixing using

deep column mixing tool

Figure 20. Graphical representation of particle

movement during mixing

5 CONCLUSIONS

The results of field & laboratory observations

have been presented for both deep column mixing

and mass mixing for UK-based projects using

wet (grout-based) techniques. For deep column

mixing, no discernible correlation was found

between sample density & strength. In addition,

the development of strength was observed to

follow the time-based progression of Åhnberg

(2006) well irrespective of whether the native soil

was predominantly granular or cohesive.

In terms of mass mixing, mixing time per unit

volume has been shown to potentially be a

governing parameter in the strength gain

progression. Field and laboratory observations

suggest that there is an optimal mixing time of

between 1.5 and 1.75 minutes per cubic metre of

mixed material. It is suggested that this parameter

should be used as a control parameter, analogous

to the blade rotation number used in deep column

mixing, for projects involving mass mixing.

It is noted that there is much scope for further

studies particularly around understanding of the

mixing mechanisms. Discrete element modelling

has shown promise in potentially setting up a

numerical framework but further development of

field & laboratory databases is also advocated.

6 ACKNOWLEDGEMENTS

The author would like to thank colleagues at

Ground Developments Ltd for data, images and

thoughtful comments & practical insights in the

preparation fo this paper.

7 REFERENCES

Munfakh, G.A. (1997) “Ground improvement

engineering – the state of the US practice: part

1. Methods”. Proc. Institution of Civil

Engineers Ground Engineering, vol. 1 pp. 193-

214

Terashi, M. and Tanaka, H. (1981) “Ground

improved by Deep Mixing Methods”, Proc.

10th ICSMFE, vol. 3, pp. 777-780

Hebib, S. and Farrell, E.R. (2004) “Stabilisation

of Irish Soils”. Inst. of Engineers of Ireland

BSI (2005) EN 14679:2005 “Execution of

Special Geotechnical Works - Deep Mixing”

British Research Establishment (2002) “Design

Guide Soft Soil Stabilisation CT97-0351

(EuroSoilStab)”

Åhnberg, H. (2006) “Strength of Stabilised Soils:

A Laboratory Study on Clays and Organic Soils

Stabilised with Different Types of Binder”

Swedish Deep Stabilisation Research Centre

Report 16

O’Brien, A. (2018) “A simplified parametric

study of particle trace in soil mixing simulations

using discrete element modelling” MSc. Thesis

School of Mathematical Sciences, University

College Cork, Ireland

Some observations on the design and construction of wet ......mechanical soil mixing and jet grouting technologies – its primary advantages being the facility to work effectively

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